Twin tunnelling induced ground settlements: A review

In the past few decades, the number of tunnels constructed next to an existing tunnel has been gradually increasing in order to accommodate infrastructure needs in congested urban cities. When a new tunnel is constructed adjacent to an existing tunnel, both the relative position of the tunnels and the construction sequence affect the ground settlement and internal forces in the linings of both tunnels. Therefore, it is important to study the influence of these factors and their relationship to tunnel construction. A review of twin tunnelling induced ground settlement is presented in this paper. A wide range of data is collected, summarized, and compared with each other to infer interaction phenomenon related to ground settlement. This data is gathered from published field observations, laboratory tests, and finite element analyses. The paper begins with an overview of single tunnelling induced settlements, volume losses, and factors which can affect twin tunnelling induced ground settlements. Next, a summary of the effects of construction sequence, pillar width, and cover depth, among other influencing factors, has been presented for four twin tunnelling configurations including (i) side-by-side, (ii) piggyback, (iii) perpendicularly crossing, and (iv) offset arrangement twin tunnelling. The paper also presents a summary of available techniques to calculate ground settlements induced by a new tunnel excavation in the presence of an existing tunnel. Finally, the paper summarizes available knowledge on ground settlement induced by various twin tunneling arrangements and identifies known unknowns.